The present invention relates in general to a method and apparatus for producing clock signals and, more particularly, to a method and apparatus for producing time advanced clock signals from an input pulse train in which the clock signals occur at a precise relationship to the position of a scanning or plotting beam.
Laser beam scanners and plotters are well known to those in the electro-optical industry. In order to accurately place a pixel on film, the laser beam must be turned on at the exact moment that the beam is over the spot where the pixel is to be written. Similarly, in order to scan an image accurately, the sampling mechanism must be activated in the same way. To accomplish this, the laser beam is split into an encoder beam and a scan (or write) beam. Both beams are moved by the same oscillating mirror that forms part of a galvonometer assembly. The encoder beam is reflected from an encoder located just above the film. The encoder, comprising alternating dark and reflective lines, auto-collimates the beam back to the galvonometer mirror and ultimately to a photo multiplier tube. The photo multiplier tube produces current pulses as the laser beam moves from reflective to non-reflective portions of the encoder. These current pulses provide the raw data for generating clocking signals for the scanners' A/D converter, memory and D/A converter. However, since the laser beam is modulated, it is necessary to compensate for the phase delay in the laser modulator.
Accordingly, it is a general object of the present invention to provide a method and apparatus for producing a time-advanced output pulse train from an input pulse train.
It is a specific object of the present invention to provide a method and apparatus for producing a time advanced output pulse train from an input pulse train utilizing a phase locked loop with time delayed feedback.
It is a further object of the invention to provide a time advanced, frequency multiplied output pulse train from an input pulse train.
It is still another object of the present invention to provide a noise free, time advanced, frequency multiplied output pulse train from an input pulse train.
It is a feature of the present invention that the method thereof can be implemented using readily available, conventional, electrical components.
It is another feature of the invention that the apparatus thereof produces clock signals occurring at a precise relationship to the position of a scanning or writing laser beam.